The invention relates to a system for injecting a fluid into a wall boundary layer of a flow in a turbomachine, a compressor and a turbomachine.
In the case of compressors in turbomachines, when operating the compressor outside a design point of the rotor blades, a so-called compressor surging may occur on a regular basis, in which the pressure in the compressor drops from the rear to the front due to a flow separation on the rotor blades and a backflow is formed. Such compressor surging has a disadvantageous effect on the efficiency of the turbomachine. In addition, high stresses occur in the blading, which are attributable to some extent to oscillating or shimming during the backflow phase. The risk of a flow separation may be reduced if a wall boundary layer of the annular space flow is energized. Energizing the wall boundary layer may be accomplished by passive systems such as stator-side ribs or depressions. On the other hand, active systems are known that provide an injection of a fluid through a plurality of nozzles disposed on the circumferential side into the annular space flow. Known active systems have nozzles with a round cross-section, whereby, however, only very limited energizing of the wall boundary layer takes place and high mixing losses also occur. An active system with rectangular nozzle cross-sections and a length/height ratio of approx. l/h=3 is known from German Patent Document No. DE 10 2008 052 372 A1. However, these types of high nozzle cross-sections produce a comparatively high retroactive effect of the annular space flow on the nozzle flow.
The object of the invention is creating a system for injecting a fluid into a wall boundary layer of a flow in a turbomachine, which eliminates the aforementioned disadvantages and makes better energizing possible as well as a reduction of mixing losses. In addition, the object of the invention is creating a compressor with a higher surge limit as well as a turbomachine with an improved efficiency.
A system according to the invention for injecting a fluid into a wall boundary layer of a flow in a turbomachine has a plurality of nozzles, which are disposed in a side wall limiting the flow and are oriented diagonally in the direction of flow. According to the invention, the nozzles each have a rectangular, flat nozzle cross-section. In particular, the nozzles have a nozzle cross-section with a length/height ratio (l/h) of approx. l/h=4 to 20. Because of the rectangular, flat and, in particular, slot-like nozzle cross-section, the so-called Coanda effect is improved and used more efficiently, whereby an improved energizing of the wall boundary layer is effected with minimal mixing losses.
Efficiency is able to be further increased if an injection channel having a constriction forming a boundary surface is disposed upstream from each of the nozzles. The constriction makes it possible for a low annular space pressure to prevail in the nozzle and therefore a pressure surge is applied away from the side wall in the nozzle. As a result, the to-be-injected fluid impacts the boundary surface perpendicularly, whereby it is coupled into the nozzle without breaking.
In order to reduce flow turbulence downstream from the constriction within the nozzles, it is advantageous if the boundary surface defines the nozzle cross-section and the fluid in the nozzles or the nozzle flow is not subject to any additional changes in pressure or speed.
The constriction may be designed so that it is beneficial for flow if the injection channel is tapered in the direction of the constriction in a wedge-shaped or funnel-shaped manner.
It may be advantageous for a further increase in efficiency if the nozzles are arranged to be flat in the direction of flow, because the fluid is hereby injected almost in the axial direction of the flow.
For example, the nozzles are arranged at an angle of ≦40° with respect to the direction of flow. They are preferably arranged at an angle of 30° with respect to the direction of flow.
In the case of an exemplary embodiment, the nozzle outlet areas are oriented tangentially to the direction of rotation, whereby the nozzle flow is injected non-rotationally into the flow.
For a rotational impact of the injected fluid, the nozzle outlet areas can be arranged with respect to the direction of rotation. In this case, they may be arranged both in the direction of rotation as well as against the direction of rotation. For example, they are oriented at an angle of 20° in or against the direction of rotation.
A compressor according to the invention has a system according to the invention for injecting a fluid into a wall boundary layer. Such a compressor is characterized by a clearly expanded surge limit and therefore by a high level of compressor stability and reduced blade stress. In addition, there is no or almost no retroactive effect of the annular space flow of the compressor on the nozzle flow of the system so that the system is able to be operated stably in every operating state of the compressor.
The compressor stability may be increased effectively already with a minimal fluid quantity if the system is positioned with its nozzles opposite from the blade regions in which a flow separation is to be expected. In the case of an exemplary embodiment, the nozzles are thus disposed on the stator side in the area of trailing edges of at least one rotor blade row formed of rotor blades.
A turbomachine according to the invention has a compressor according to the invention. Such a turbomachine is characterized by low efficiency losses and therefore by a high degree of efficiency, because energizing the wall boundary layer is improved and mixing losses are reduced.
Preferred exemplary embodiments of the invention will be explained in greater detail in the following on the basis of the very simplified schematic representations.